This application claims the priority of German application 198 47 210.2, filed in Germany on Oct. 13, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for testing valve disks of ceramic charge cycle valves for internal-combustion engines as well as to an arrangement for carrying out the method.
During mechanical strain, ceramic materials fail at defective spots, which may be natural sintering defects as well as defects caused by processing, such as grinding cracks, or thermally induced cracks. Depending on the type of strain, surface defects or volume defects are decisive with respect to the failure.
For reasons of cost, inlet and outlet valves made of silicon nitride (Si3N4), during the manufacturing from the sintered blank by means of diamond wheels, must be ground at a high cutting speed and cutting capacity. This means that defects are produced in the surface and in the edge zone close to the surface. These may be cracks, shattered portions of material and deep chatter marks. The defects are only partially removed during the finishing grinding. As a rule, grinding cracks remain in the material. During the operation, the failure of ceramic inlet and outlet valves often takes place in the valve disk edge area. The targeted testing of the valve disk edge after the manufacturing of the valves is therefore a priority of quality assurance. It cannot be carried out by means of nondestructive testing methods or can be carried out only at very high expenditures.
From Japanese Patent Document JP 3-264,713 A, it is known to test ceramic valves by means of a tension test. For this purpose, the valve disk, on the one hand, and the valve stem, on the other hand, are clamped into corresponding receiving devices.
It is therefore an object of the invention to provide a testing method and/or an arrangement by means of which ceramic valves can be tested more easily; particularly the breaking action and the breaking origin can be observed more easily; and thus defects caused by processing can be determined with respect to their extent and effect by measuring the strength.
According to preferred embodiments of the invention, this object is achieved by a method for testing valve disks of ceramic charge cycle valves for internal-combustion engines, comprising selecting three stressing points within half the circumference of the valve disk, introducing a pressure by means of stressing devices into two of the stressing points provided on a valve seat side of the valve disk, and introducing a pressure by means of a stressing device into the third stressing point provided on a side of the valve disk facing away from the valve seat, wherein the stressing points are situated in a radially exterior area of the valve disk in the area of the valve seat, and wherein one of said stressing points is placed between the two other stressing points.
Thus, a valve disk is stressed with respect to bending along a portion of its edge area which is smaller than half the circumference such that the truncated-cone-shaped valve seat side of the valve disk is stressed under tension. In this case, a pressure is exercised by means of a stressing device particularly in the axial direction at a first stressing point of the valve disk on the side of the valve disk situated opposite the valve seat, and a counterpressure is exercised at two stressing points on the valve seat side. Viewed in the circumferential direction, the two stressing points of the valve seat side are situated on this side and on the opposite side of the stressing point provided on the side facing away from the valve seat. The three stressing points are situated within an area which corresponds to half the circumference of the valve disk.
The three stressing points may preferably be situated within a narrower area which corresponds to one sixth to one twelfth of the circumference.
In this manner, this area of the valve disk is locally stressed by bending, in which case the valve seat side is stressed under tension so that a failure of the valve disk is caused which is close to reality and can be reproduced.
Instead of cones or balls, which produce punctiform pressure points, cylindrical or truncated-cone-shaped rollers can be used as stressing devices, which introduce the load by way of a surface line into the valve disk. The stressing points will then be contact lines. These contact lines are preferably radially aligned on the side facing away from the valve seat.
The two contact lines on the valve seat side may extend mirror-symmetrically to one another with respect to an axial plane extending through the first contact line. However, a radial alignment of these contact lines is also contemplated.
On the diametrically opposite, circumferential side of the valve disk, stops are situated for the radial holding or fixing of the valve disk in relation to the stressing devices.
If the pressure points are designed correspondingly, the two contact lines on the valve seat side may be shorter than the width of the disk edge.
The method is implemented by means of an arrangement including a testing machine with a support for the valve disk and a pressure foot for exercising a pressure force onto the valve disk. In preferred embodiments of the invention, this bending test arrangement has a support and a pressure foot wherein, on the valve seat side of the valve disk, the support is equipped with two stressing rollers mirror-invertedly arranged with respect to an axial plane. A stop roller is situated on the side diametrically opposite the two stressing rollers.
Additional advantages, characteristics and details of the invention are contained in the claims as well as in the following description, in which a particularly preferred embodiment is described in detail with reference to the drawing. In this case, the characteristics illustrated in the drawing and indicated in the claims as well as in the description may in each case be essential to the invention individually or in any combination.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.